Satellites Record Weakening North Atlantic Current Impact

A North Atlantic Ocean circulation system weakened considerably in
the late 1990s, compared to the 1970s and 1980s, according to a NASA
study.

Sirpa Hakkinen, lead author and researcher at NASA's Goddard Space
Flight Center, Greenbelt, Md. and co-author Peter Rhines, an oceanographer
at the University of Washington, Seattle, believe slowing of this ocean
current is an indication of dramatic changes in the North Atlantic
Ocean climate. The study's results about the system that moves water
in a counterclockwise pattern from Ireland to Labrador were published and can be found on the Science Magazine website.

The current, known as the sub polar gyre, has weakened in the past
in connection with certain phases of a large-scale atmospheric pressure
system known as the North Atlantic Oscillation (NAO). But the NAO has
switched phases twice in the 1990s, while the subpolar gyre current
has continued to weaken. Whether the trend is part of a natural cycle
or the result of other factors related to global warming is unknown.

"It is a signal of large climate variability in the high latitudes," Hakkinen
said. "If this trend continues, it could indicate reorganization
of the ocean climate system, perhaps with changes in the whole climate
system, but we need another good five to 10 years to say something
like that is happening." Rhines said, "The subpolar zone
of the Earth is a key site for studying the climate. It's like Grand
Central Station there, as many of the major ocean water masses pass
through from the Arctic and from warmer latitudes. They are modified
in this basin. Computer models have shown the slowing and speeding
up of the subpolar gyre can influence the entire ocean circulation
system."

Satellite data makes it possible to view the gyre over the entire
North Atlantic basin. Measurements from deep in the ocean, using buoys,
ships and new autonomous "robot" Seagliders, are important
for validating and extending the satellite data. Sea-surface height
satellite data came from NASA's Seasat (July, August 1978), U.S. Navy's
Geosat (1985 to 1988), and the European Space Agency's European Remote
Sensing Satellite1/2 and NASA's
TOPEX/Poseidon (1992 to present).

Hakkinen and Rhines were able to reference earlier data to TOPEX/Poseidon
data, and translate the satellite sea-surface height data to velocities
of the subpolar gyre. The subpolar gyre can take 20 years to complete
its route. Warm water runs northward through the Gulf Stream, past
Ireland, before it turns westward near Iceland and the tip of Greenland.

The current loses heat to the atmosphere as it moves north. Westerly
winds pick up that lost heat, creating warmer, milder European winters.
After frigid Labrador Sea winters, parts of the water mass in the current can become cold and dense enough to sink beneath the surface, and head slowly southward back to the equator. The cycle is sensitive to the paths
of winter storms and to the buoyant fresh water from glacial melting
and precipitation, all of which are experiencing great change.

While previous studies have proposed winds resulting from the NAO
have influenced the subpolar gyre's currents, this study found heat
exchanges from the ocean to the atmosphere may be playing a bigger
role in the weakening current. Using Topex/Poseidon sea-surface height
data, the researchers inferred Labrador Sea water in the core of the
gyre warmed during the 1990s. This warming reduces the contrast with
water from warmer southern latitudes, which is part of the driving
force for ocean circulation.

The joint NASA-CNES (French Space Agency) Topex/Poseidon oceanography
satellite provides high-precision data on the height of the world's
ocean surfaces, a key measure of ocean circulation and heat storage
in the ocean.

NASA's Earth Science Enterprise is dedicated to understanding the
Earth as an integrated system and applying Earth System Science to
improve prediction of climate, weather and natural hazards using the
unique vantage point of space. NASA, the National Oceanic and Atmospheric
Administration, and the National Science Foundation funded the study.

Dominant Pattern of Variability of the Sea-surface
Height in the 1990s. This image shows the dominant pattern of variability
of the sea-surface height in the 1990s. The slope of the sea-surface
height is in balance with ocean currents, much in the way weather maps
of pressure relate to winds. The large blue region in the northern
Atlantic represents a slowing of the counter-clockwise, cyclonically
circulating subpolar gyre. For this image, the researchers used TOPEX/Poseidon
data, which has been combined with ERS-1/2 data into the NASA Pathfinder
data set. The Pathfinder data set also includes the Seasat and Geosat
data which are referenced to TOPEX/Poseidon data. The next few years
will reveal how sea surface height is going to evolve as the altimetric
time series continues with JASON-1 observations.
Credit: Sirpa Hakkinen,
NASA GSFC

Terra MODIS Sea Surface Temperatures for North
Atlantic Ocean. This image of North Atlantic Ocean sea surface temperatures
represents an eight-day composite from Sept 6 - Sept 13, 2001 from
the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument
on Terra. The red and orange colors represent warmer water, while
the blue shades represent cold water in the higher latitudes. The
Gulf Stream is evident along the U.S. eastern seaboard. The cold water region with very cold coastal waters along the Greenland and Labrador coasts form the western part of the subpolar gyre circulating anticlockwise. The color
bar, in enlarged version, is in units of degrees Celsius.
Credit: Ronald Vogel, SAIC for
NASA GSFC

Iceberg in North Atlantic Waters. An iceberg glides
southward along the east coast of Greenland in the Irminger Sea. The
tip of this iceberg is about the size of a 2-story house.
Credit: C.
A. Linder. For a high resolution version of this image please contact
Christopher Linder at http://www.chrislinder.com

Trend Velocities in North Atlantic. The trend of the
velocities (meters per second per decade) derived from NASA Pathfinder
altimeter data for the period May 1992 to June 2002. The colored vectors
are statistically significant. Note how the vectors trace the following
graphic of the subpolar circulation in reverse direction, which denotes
a slowing gyre. The colors refer to t-test values (where anything above 2 is considered significant to 95%).
Credit: Sirpa Hakkinen, NASA GSFC